A variety of different types of radio paging systems are in widespread use. For example, a radio paging system that uses a variation of the well known POCSAG protocol is shown in U.S. Pat. No. 4,398,192 (Moore) and a system that broadcast messages using an FM sub-carrier is shown in U.S. Pat. No. 4,713,808 (Gaskill).
None of the know systems is absolutely reliable, that is, none of the known systems can guarantee that one hundred percent of the messages transmitted will be received by the intended receiver. Problems associated with message reception reliability are accentuated when the size of the receivers is decreased. For example, the wristwatch like receiver shown in U.S. Pat. No. 4,7113,808 (Gaskill) imposes sever restrictions on battery size and on the antenna, thereby creating sensitivity or message completion reliability problems.